Backplanes are the brains of displays. Amorphous silicon (a-Si) is used for over 90% of flat panel displays (FPD) even to this day, and has established a solid manufacturing infrastructure including the technology advancements that has developed in the past decade. Due to its very low electron mobility (1 cm2/V-s), it is not suitable for high performance LCDs and OLED displays in today's mobile devices with higher resolution TFT-LCD and AMOLED.
Low Temperature Polycrystalline Silicon (LTPS) can be used as the available backplane technology for these applications. LTPS is built on the a-Si technology in several respects, such as LTPS is formed by low temperature re-crystallization of a-Si. In addition, LTPS can have many commonalities with a-Si, such as device design, processing, reliability historical data, and potentially low costs. Its electron mobility is about 50 cm2/V-s, much higher that of a-Si.
High resolution TFT-AMLCD or AMOLED displays can require high mobility transistors, which can be fabricated using Low Temperature Poly-Silicon (LTPS). The fabrication of LTPS backplanes requires a laser annealing step. LTPS backplanes can require additional transistors to correct the non-uniformity caused by the finite grain size of silicon in the LTPS. Another limitation of LTPS is the high off-state leakage current, also caused by the presence of grain boundaries. Voltage drift over time, again attributed to the presence of grain boundaries in LTPS, is another problem associated with LTPS. Thus it can be difficult to develop an economical process for making LTPS transistor arrays.
Recently, new processes and materials have been developed. For example, Indium Gallium Zinc Oxide (IGZO) can be used as the channel material for the transistors in the back plane. Since indium and gallium are scarce and expensive materials, IGZO-based back plane can be expensive.
SiOG process, which includes a single crystal silicon layer on a glass substrate, has been proposed for use as backplane for high performance mobile display applications. Its high electron mobility, greater than 500 cm2/V-s, attributable to its single crystal nature, will be a great benefit to backplane applications. The single crystal silicon-on glass substrate can be prepared by first implanting hydrogen into a silicon wafer with sufficient energy and fluence, forming a defect plane that can be used to exfoliate the top portion of the wafer. A glass sheet can be attached to the implanted wafer, for example, by stiction at room temperature. The composite structure can be heated, for example, on a hot plate, to exfoliate a thin silicon layer attached to glass. The bonding of the thin silicon to the glass substrate can be strengthened by heating to a higher temperatures or by anodic bonding.
SIOG shows promises for backplane applications, providing all the expected benefits of “a single crystal CMOS-type TFT” for OLED displays, including the benefits of electron mobility in excess of 500 cm2/V-s, together with the ability to integrate drivers and other circuits, lower power consumption, greater resolution, higher speed, better uniformity, lower nose, higher brightness displays. SiOG transistors can be better than LTPS transistors in all of the above characteristics, and can be nearly equivalent to those made on SOI.
A potential problem for SIOG transistors is its high current leakage in the OFF position, e.g., higher by two orders of magnitude as compared to a-Si or LTPS transistors. Its behaviors can also be worse in terms of 1/f noise as compared to SOI.